Various devices are known for forming a seal between a rotatable shaft (or a sleeve or runner mounted on a rotatable shaft) and a stationary housing or other structure surrounding the shaft. Some seals make contact with the shaft to minimize leakage and may be referred to as “contact circumferential shaft seals.” These seals include one or more seal rings with circumferential inner faces that contact the rotating sleeve and slide against the sleeve while it rotates. Such seals may be formed from compacted and sintered carbon graphite to provide heat and wear resistance, and they are often formed as a plurality of interconnectable ring segments to facilitate installation about the sleeve. The seal rings are held in place by a suitable retaining device, and a seal including such rings may include biasing devices, such as a circumferential or garter spring, for holding the seal segments together, and a plurality of axial compression springs to encourage side seating of the seal segments against the stationary housing.
While contact seals are durable and capable of withstanding high levels of heat and friction, sliding contact with a rotating sleeve eventually causes the seal rings to wear out. The rate at which the carbon rings wear is based in part on the relative speed of the sleeve and shaft, and in some high-efficiency jet engines, this speed, expressed as a linear velocity, can exceed 600 feet/second or about 400 miles/hour, for extended periods of time. The heat generated by contact at such speeds causes the seal rings to wear, and the rings therefore require frequent maintenance and/or replacement. The desire for longer operating life and higher thermal efficiency has moved the seal industry to look for alternatives to circumferential contact seals.
A solid bushing seal is one alternative to a circumferential contact seal. As the name implies, these seals are solid or unitary, and they avoid the problem of wear by maintaining a small spacing or gap between the shaft and the seal. However, shafts on which seals are used often expand due to thermal expansion and/or centrifugal force during use. In order to avoid damaging a solid bushing seal, the inner diameter of the seal must be large enough to remain spaced from the expanded shaft. This need to provide a gap results in a relatively large leakage rate at start up, when the shaft is cool. Moreover, if the gap is not large enough, the shaft may crack or otherwise damage the seal when it heats up and expands.
Another alternative to contact seals is the circumferential gas film seal. Much like the circumferential contact seal, this seal includes one or more carbon seal rings that exert a very light contact force against the shaft or sleeve when it is rotating and when it is not. The light contact force is achieved by routing high pressure gas to opposing faces through clearance spaces and milled cutouts. In the case of a contacting circumferential seal, the outer diameter of the ring is exposed across its entire width while the inside diameter is exposed across its entire width except for the width of a small sealing dam. This creates an imbalance in force that lightly seats the seal against the rotating sleeve. Producing a force balanced contact in this manner is referred to hydrostatic sealing, and a hydrostatic seal can be maintained both when the shaft is rotating and when the shaft is stationary.
Alternately or in addition, hydrodynamic sealing can be produced by forming recesses or cutouts on the side of the seal ring that faces the sleeve. As the sleeve rotates, air entrained by the rotating sleeve is compressed in these cutouts, and as it escapes over the non-recessed “pads” between the recesses, it produces an additional pressure and flow of air for maintaining a separation between the seal ring and the sleeve. A circumferential gas film seal is disclosed in co-pending U.S. patent application Ser. No. 14/132,571, “Bidirectional Lift-Off Circumferential Shaft Seal Segment and A Shaft Seal Including A Plurality of the Segments,” the contents of which application is hereby incorporated by reference. Circumferential gas film seals generate less friction and less heat than circumferential contact seals, and thus generally last longer, require less maintenance and experience less oil cooling efficiency loss than contact seals.
There is an increasing demand for seals that can operate at higher temperatures and pressures. Modern jet engine designs require more robust seals that can operate at higher speeds, temperatures and pressures than ever before, and many conventional seals rapidly degrade under these conditions. Gas film seals in particular tend to suffer damage when used with a rotating shaft that is operated at the top end of its design range. Alternately, insufficiently robust seals prevent an engine from operating over its full range for fear of damaging the shaft seals.
At high speeds and pressures, the rotating shaft tends to rub against and damage the seal despite presence of the cutouts discussed above. That is, the seal is no longer able to maintain an adequate air film for supporting the seal, and the rotating shaft comes into contact with the seal. The geometry of the radially inner seal face changes as it wears and thereafter forms a less effective seal with the shaft surface when it drops back onto the shaft surface when the shaft is stopped. Repeatedly subjecting the shaft/seal interface to extreme conditions continues to damage the radially inner surface of the seal, and the damaged seal may eventually fail catastrophically when the geometry of the inner radial surface of the seal is no longer sufficient generate lift and keep the seal off the shaft.